During her three decades of working with elephant seals, Dr. Marcela Uhart had never seen anything like the scene on the beaches of Argentina’s Valdés peninsula last October.
It was peak breeding season; The beach should have been teeming with harems of fertile females and huge males fighting for dominance. Instead, it was “just cadaver upon cadaver upon cadaver,” Dr. recalls. Uhart, who directs the Latin American wildlife health program at the University of California, Davis.
H5N1, one of the many viruses that cause bird flu, has already killed at least 24,000 South American sea lions on the continent’s coasts in less than a year. Now it was the elephant seals.
Puppies of all ages, from newborns to fully weaned animals, lie dead or dying at the flood line. Sick puppies lay listless, foam oozing from their mouths and noses.
Dr. Uhart called it “a picture from hell.”
In the weeks that followed, she and a colleague – protected from head to toe with gloves, gowns and masks and regularly doused with bleach – carefully documented the devastation. Team members stood on nearby cliffs and used drones to determine the toll.
What they found was shocking: The virus had killed an estimated 17,400 seal pups, more than 95 percent of the colony’s pups.
The disaster was the latest in a bird flu epidemic that has raged around the world since 2020, prompting authorities on several continents to kill poultry and other birds by the millions. In the United States alone, more than 90 million birds have been killed in a vain attempt to deter the virus.
H5N1 could not be stopped. Avian flu viruses tend to be selective about their hosts and typically stick to one species of wild bird. But this bird has quickly nested in a surprisingly wide variety of birds and animals, from squirrels and skunks to bottlenose dolphins, polar bears and, most recently, dairy cows.
“In my flu career, we have never seen a virus expand its host range like this,” said Troy Sutton, a virologist who studies avian and human influenza viruses at Penn State University.
The blow to marine mammals, as well as the dairy and poultry industries, is worrying enough. What is even more worrying, experts say, is what these developments suggest: the virus is adapting to mammals and is moving ever closer to spreading among humans.
A human pandemic is by no means inevitable. At least so far, the changes in the virus do not suggest that H5N1 can cause a pandemic, said Dr. Sutton.
Still, he said, “We really don’t know how to interpret this or what it means.”
Marine mortality
A highly pathogenic H5N1 strain was identified in domestic waterfowl in China in 1996. The following year, 18 people in Hong Kong were infected with the virus and six died. The virus then fell silent, but resurfaced in Hong Kong in 2003. Since then, it has caused dozens of outbreaks in poultry and affected more than 800 people who were in close contact with the birds.
All the while it was evolving.
The current global version of H5N1 emerged in Europe in 2020 and quickly spread to Africa and Asia. It killed numerous farm birds, but unlike its predecessors, it also spread widely among wild birds and many other animals.
Most mammalian infections were probably “dead-end” cases: perhaps a fox that ate an infected bird and died without passing on the virus. However, some major outbreaks suggested that H5N1 is capable of more.
The first clue came in the summer of 2022, when the virus killed hundreds of seals in New England and Quebec. A few months later, it infiltrated a mink farm in Spain.
At least in the case of minks, the most likely explanation was that H5N1 had adapted to spread among the animals. The scale of marine mammal outbreaks in South America underscores this likelihood.
“Even intuitively, I would think that mammal-to-mammal transmission is very likely,” said Malik Peiris, a virologist and bird flu expert at the University of Hong Kong.
After it was first detected in South America in birds in Colombia in October 2022, the virus spread along the Pacific coast to Tierra del Fuego, the southernmost tip of the continent, and then up the Atlantic coast.
Along the way, it killed hundreds of thousands of seabirds and tens of thousands of sea lions in Peru, Chile, Argentina, Uruguay and Brazil. The sea lions behaved erratically and suffered from convulsions and paralysis; Pregnant females miscarried their fetuses.
“What happened when the virus got to South America was something we had never seen before,” said Dr. Uhart.
Exactly how and when the virus jumped to marine mammals is unclear, but the sea lions most likely came into close contact with infected birds or contaminated feces. (Although fish make up the majority of sea lions’ diet, they also sometimes eat birds.)
It is likely that at some point the virus evolved to spread directly among marine mammals: in Argentina, sea lion deaths did not coincide with mass deaths of wild birds.
“This may indicate that the source of infection was not the infected birds,” said Dr. Pablo Plaza, wildlife veterinarian at the National University of Comahue and the National Scientific and Technical Research Council in Argentina.
It’s not hard to imagine how the virus might spread among these animals: Elephant seals and sea lions both breed in colonies and huddle together on beaches, where they fight, mate and bark at each other. Elephant seals sneeze all day long, expelling large droplets of mucus each time.
It is difficult to prove exactly how and when the virus jumped from one species to another. However, genetic analysis supports the theory that marine mammals transmitted their infections from each other rather than from birds. Virus samples isolated from sea lions in Peru and Chile and from elephant seals in Argentina all have about 15 mutations not seen in the birds; The same mutations also occurred in a Chilean man who was infected last year.
There are numerous ways for H5N1 to jump from marine mammals to humans. A sick male elephant seal that sat on a public beach in Argentina for a day and a half was found to be carrying massive amounts of viruses. In Peru, scientists collected samples from sea lion carcasses lying next to families enjoying a day at the beach.
Scavenging animals like dogs could also pick up the virus from an infected carcass and then spread it further: “None of the wild animals exist in their little silos,” said Wendy Puryear, a virologist at Tufts University who has studied New England seal outbreaks.
In some South American countries, apart from a few buried carcasses, the rest are left on the beaches, rotting and looted.
“How do you even manage to dispose of 17,000 bodies in the middle of nowhere, in places where you can’t even bring down machines, and on huge cliffs?” said Dr. Uhart.
A mutating pathogen
Flu viruses are adept at recognizing new mutations; When two types of flu viruses infect the same animal, they can mix their genetic material and create new versions.
It is unclear exactly how and how much the H5N1 virus has changed since it first appeared. A study last year showed that after the virus entered the United States, it quickly mixed with other flu viruses circulating here and morphed into different versions – some mild, others causing severe neurological symptoms.
“Now, after 20 years of rearrangement, you have a virus that actually works exceptionally well in a whole range of bird and mammal species,” said Vincent Munster, a virologist at the National Institute of Allergy and Infectious Diseases who studies the required mutations has so that H5N1 can adapt to humans.
Each new species that harbors the virus provides an opportunity for H5N1 to evolve and invade humans.
And the virus can encounter mutations that no one has yet thought of, breaking the species barrier. This happened during the swine flu outbreak in 2009.
This virus did not have the mutations thought to be necessary to easily infect people. Instead, “there were these other mutations that no one knew about or had thought of until then,” said Louise Moncla, an evolutionary biologist who studies bird flu at the University of Pennsylvania.
But even if the virus jumps to people, “we may not see the mortality rate that really concerns us,” said Seema Lakdawala, a virologist at Emory University. “Pre-existing immunity to seasonal flu strains provides some protection against serious illness.”
What happens next
The U.S. is prepared for a flu pandemic with some supplies of vaccines and antivirals, but its efforts to monitor the virus may not move quickly enough to deploy those tools.
It took several weeks for farmers and then authorities to learn that H5N1 was circulating in dairy cows.
The outbreak on dairy farms has resulted in only a mild infection in humans, but farms are fertile ground for the virus, which jumps to species — from cat to cow to pig to human, in any order.
Many scientists are particularly concerned about pigs, which are susceptible to both human and bird flu strains and represent the perfect mixing bowl for viruses to swap genes. Pigs are slaughtered at a very young age, and younger generations who have never been exposed to flu are particularly vulnerable to infection.
So far, H5N1 does not appear to be able to infect pigs, but that could change as it acquires new mutations.
“I never let my children go to a state fair or an animal farm, I am one of those parents,” said Dr. Lakdawala. “And that’s mostly because I know that the more interactions we have with animals, the more opportunities there are.”
Should H5N1 adapt to humans, federal officials will need to work with their international counterparts. Nationalism, competition and bureaucracy can slow information sharing, which is crucial as an outbreak develops.
In some ways, the current spread among dairy cows is an opportunity to practice the drill, said Rick Bright, managing director of Bright Global Health, a consulting firm focused on improving the response to public health emergencies. But the U.S. Department of Agriculture only requires voluntary testing of cows and is not as timely or transparent with its results as it should be, he said.
Dr. Rosemary Sifford, the department’s chief veterinarian, said staff there are working hard to share information as quickly as possible. “This is considered an emerging disease,” she said.
Government leaders are typically cautious and want to see more data. But “given the rapid pace at which this disease can spread and the devastating disease it can cause if our leaders hesitate and fail to pull the right triggers at the right time, we are once again caught flat-footed,” said Dr. Bright said.
“If we show it not panic but respect and due care,” he added, referring to the virus, “I believe we can do it.”
Source link
2024-04-24 23:35:48
www.nytimes.com
